The present invention relates to measurement devices and, more particularly, to electronic devices for measuring the consumption of electricity by a load.
In my prior U.S. Pat. Nos. 3,875,508; 3,875,509; among others. I disclose power consumption measurement techniques in which an analog signal representative of one of load current and voltage is pulse-width modulated by a signal representative of the other of load current and voltage. The resulting pulse-width-modulated signal is the product of instantaneous voltage and current. The product signal contains an oscillating component, which is filtered therefrom, and a desired average component which is integrated for application to succeeding circuits.
Concerns for reliability, low power consumption and low-cost manufacturing make desirable the use of integrated circuits to perform as many metering functions as possible. External devices required by the metering devices of my prior disclosures such as, for example, relays, resistors, capacitors and inductors, prevent substantially full integration thereof on a silicon integrated circuit. Current CMOS (complementary metal oxide semiconductor) technology is capable of providing switching and amplifying functions on a single silicon chip without the need for external components.
The integrating function employed to separate the average component from the oscillating component of the product signal conventionally requires resistors and capacitors of high accuracy. Available CMOS techniques are incapable of producing resistors and capacitors on the silicon chip having values well enough controlled to attain required measurement accuracies. For example, on-chip resistors exhibit poor temperature stability. As a consequence, a CMOS electronic metering device requires external resistors and/or capacitors. This increases manufacturing cost and reduces product reliability. Component tolerances of the external components may require final adjustment during manufacture to attain the desired measurement accuracy. In addition, the ability of such CMOS electronic metering devices with external components to maintain calibration accuracy throughout the wide temperature range to which conventional watthour meters are subjected, is degraded.
One type of integrating device, disclosed in the following papers, includes a switched-capacitor integrator especially adapted for realization in a metal-oxide semiconductor integrated circuit: "Potential of MOS Technologies for Analog Integrated Circuits"; david Hodges, Paul Gray and Robert Broderson; IEEE Journal Solid-State Circuits, June 1978, pages 285-294. "MOS Sampled Data Recursive Filters Using Switched Capacitor Integrators"; Bedrick Hostika, Paul Gray and Robert Broderson; IEEE Journal Solid-State Circuits, June 1987, pages 600-608. "Effect of Switch and Routing Related Parasitic Capacitances"; Modern Filter Design, pages 458-461, Prentice Hall.
The above papers disclose filters, integrators, and analog-to-digital converters, integrated on a single chip using a switched capacitor to replace the input resistor of an integrator. The time constant of the switched-capacitor integrator is equal to the ratio of the integrating capacitor divided by the clock frequency. Since a given ratio of two capacitors formed on the same silicon chip is easy to attain, and since the temperature coefficients of such capacitors tend to track each other very closely, many of the drawbacks of the prior-art integrators are overcome.
A further problem in prior-art electronic watthour metering devices is caused by offset voltages in amplifiers and threshold devices used therein. In my referenced patents and patent application, I disclose a technique for integrating alternately upward and downward between positive and negative threshold voltages. Any existing offset voltage adds to the signal during one direction of integration and subtracts therefrom during integration in the other direction. This cancels the effect of the offset voltage.
The following papers disclose techniques for periodically storing an image of the offset voltage and for applying the image to cancel the effect thereof: "Offset-Compensated Switched-Capacitor Leapfrog Filters"; S. Eriksson, K. Chen; Electronic Letters, pages 731-733; August, 1984. "Techniques for Offset Voltage Cancellation in MOS Operational Amplifiers"; S. Wong, C. Salama; Electronic Letters, pages 389-390; April, 1985.
None of the foregoing references addresses the problem of an integrated electronic watthour metering device.